[meta] tell users what configuration their tests are running with

[sunshowers]

Nextest has a number of sources of configuration:

• repository-specific configs
• probably user-specific configur
• CLI and environment parameters
• profiles and per-test overrides

As the complexity of configuration options grows, we should have ways to make it clear to users how their tests are actually running.

This meta-level issue encompasses both human and machine-readable output.

• Human-readable configuration output #1526
• Machine-readable configuration output #1527